Matho. But pray let me interrupt you to aſk why the ſoap-bubbles, which boys have ſuch a pleaſure in blow-up, that they may behold the ſtrange variety of fine colours in them, are all of this ſpherical figure, whether they be great or ſmall?
Philon. Theſe bubbles, Matho, are not the diverſion of boys only, but alſo of philoſophers; and this their ſpherical figure is owing to a different cauſe.
M. To what different cauſe?
P. It is owing to the mutual repulſe between the particles of air: for this contrary affection ſerves great purpoſes in nature, and proceeds from the power of the ſame immaterial cauſe. (Matho; or, The Coſmotheoria Puerilis: A Dialogue, 1765)

Both children and adults blew bubbles in the 18th century for fun as well as to experiment with scientific principles. While clean tobacco-pipes were sometimes used for this function, many also used long, thin hollow straws, as in this description from The Young Exiles:

After the muſic lady Charlotte brought in a large bowl full of ſoap-ſuds, together with ſome ſtraw-pipes, requeſting my ſiſter to get up on a chair and blow bubbles, in order that ſhe might again behold her in the ſame attitude in which ſhe had firſt obtained admiſſion to her at Mrs Purvis’s. My ſiſter replied that ſhe was grown much older and much taller ſince that time: nevertheleſs, ſhe blew ſoap-bubbles with a ver good grace; and all the company did the ſame, not excepting even ſenhor Xavier.

The vessels used to hold the soap suds also vary considerably among the illustrations. Many use bowls, but glasses and scallop-shells also appear to have been used for this purpose.

See also 17th century bubble-blowers by Mieris, Mignard, Naiveu, and Netscher.